remove plural marker on abstract non-count noun

"Overhead" is an abstract non-count noun and thus it is not normally possible to pluralize it in English.  In this respect it is like "information" or "happiness".

I was not sure which file to make this change in, as I don't know which files are derived from which.  I've you'd like, I can make the change in another location—just let me know where it makes the most sense.

I hope this is helpful.

Author: brandonzylstra

doc/content/en/docs/_index.md

@@ -39,7 +39,7 @@ Avro provides:
 
 ## Schemas
 
-Avro relies on schemas. When Avro data is read, the schema used when writing it is always present. This permits each datum to be written with no per-value overheads, making serialization both fast and small. This also facilitates use with dynamic, scripting languages, since data, together with its schema, is fully self-describing.
+Avro relies on schemas. When Avro data is read, the schema used when writing it is always present. This permits each datum to be written with no per-value overhead, making serialization both fast and small. This also facilitates use with dynamic, scripting languages, since data, together with its schema, is fully self-describing.
 
 When Avro data is stored in a file, its schema is stored with it, so that files may be processed later by any program. If the program reading the data expects a different schema this can be easily resolved, since both schemas are present.